Residual Stresses and Fatigue of Surface Treated Welied Specimens

An e x p e r i m e n t a l program was under t aken t o e v a l u a t e t h e e f f e c t of induced compress ive s t r e s s e s due t o v a r i o u s peen ing and t h e r m a l t r e a t m e n t s on t h e f a t i g u e l i f e of a b u t t welded medium s t r e n g t h s t e e l (Cb-50). R e s i d u a l s t r e s s e s were measured u s i n g x-ray d i f f r a c t i o n and t h e r e s u l t s were compared w i t h h a r d n e s s v a l u e s t a k e n a t t h e s u r f a c e and t h r o u g h t h e t h i c k n e s s o f t h e specimens. I t was found t h a t of a l l p o s t weld t r e a t m e n t s s t u d i e d t h e s t r e s s peened specimens gave t h e most improvement i n f a t i g u e s t r e n g t h o v e r t h a t of t h e as-welded c o n d i t i o n and approached 82% of t h e f a t i g u e s t r e n g t h of t h e b a s e m e t a l a t a f a t i g u e l i f e of 2 x l o 6 c y c l e s . On t h e o t h e r hand a n n e a l i n g o n l y showed a n improvement of 14% over t h a t of t h e as-welded c o n d i t i o n . It was found t h a t a good l i n e a r c o r r e l a t i o n e x i s t e d between t h e induced r e s i d u a l s t r e s s e s a t t h e t o e of t h e weld and t h e f a t i g u e s t r e n g t h e v a l u a t e d a t 2 x l o 6 c y c l e s . However, f o r r e s i d u a l s t r e s s e s of l e s s t h a n 130 MPa v e r y l i t t l e improvement i n f a t i g u e s t r e n g t h was e v i d e n t . S u r f a c e r e s i d u a l s t r e s s e s were compared w i t h Almen s t r i p r e a d i n g s and t h e d e p t h of work ha rden ing . I t was found t h a t t h e Almen s t r i p r e a d i n g s d i d n o t g i v e a s c o n s i s t e n t a n i n d i c a t i o n of work ha rden ing a s d i d measurement of t h e r e s i d u a l s t r e s s e s .